This invention relates generally to a heat and pressure fuser for an electrophotographic printing machine, and more particularly the invention is directed to a device for measuring the surface temperature of a heated fuser member.
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to selectively dissipate the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an adhesive material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I. DuPont De Nemours under the trademark Teflon. In these fusing systems, however, since the toner image is tackified by heat, it frequently happens that a part of the image carried on the supporting substrate will be retained by the heated fuser roller (commonly known as hot offset) and not penetrate into the substrate surface. The tackified toner may stick to the surface of the fuser roll and offset to a subsequent sheet of support substrate or offset to the pressure roll when there is no sheet passing through a fuser nip resulting in contamination of the pressure roll with subsequent offset of toner from the pressure roll to the image substrate.
In order to insure that such a fuser functions at the desired operating temperatures, a thermistor or some other type temperature controlling device is employed. Conventionally, the temperature controlling device or devices physically engage the surface of the fuser member which most commonly comprises of a pair of roller members. One such fuser utilizes an internally heated fuser roll. As will be appreciated, the heated fuser member may be heated using an external source.
The most common type of temperature sensor or controller is a bead thermistor. While such a device is quite satisfactory for use in connection with black monochrome printing, use of the bead thermistor in color imaging is not satisfactory. This is because the pressure required for proper engagement of the bead thermistor with the roll surface causes, not only wear of the fuser roll surface, but it also results in image gloss variation due to the change in roll surface properties caused by such wear.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, may provide a better understanding and appreciation of the present invention.
U.S. Pat. No. 5,281,793 granted to Gain et on Jan. 25, 1994 discloses an apparatus for positioning a temperature sensing element in temperature sensing relationship with a moving object. The apparatus includes a base and also a resilient member, attached to the base, for supporting the sensing element. The apparatus further includes a mechanism for urging the sensing element toward the moving object. Moreover, the apparatus includes a mechanism, interposed between the resilient member and the moving object, for reducing frictional resistance between the apparatus and the moving object. The apparatus additionally includes a mechanism, removable attachable to the base, for securing the resistance reducing mechanism at a position interposed between the resilient member and the moving object. In this patent, a portion of a tape is interposed between a resilient member and a fuser roll for minimizing friction therebetween. The tape is fabricated from a polyimide film. By way of example, tape can be made from KAPTON, a trademark of E. I. duPont de Nemours & Co., Inc. of Wilmington, Del.
U.S. Pat. No. 5,194,890 granted to Haruna et al on Mar. 16, 1993 discloses a temperature sensor is fixed on the copying apparatus so that it makes contact with the surface of the heating roller which is coated with Teflon (tetrafluoroethylene). The temperature sensor is covered with KAPTON resin to improve the wear-resistance.
U.S. Pat. No. 3,914,862 granted to McBride et al discloses a method of making a sensor wherein a cylindrical KAPTON sleeve used for insulating the side walls of a can.
Japanese publication 59-44633 dated Mar. 13, 1984 discloses a temperature detector for a fixing roll wherein a polyimide sheet is made to adhere as one body to a plastic substrate.
U.S. Pat. No. 4,821,062 granted to Katoh et al on Apr. 11, 1989 discloses a thermistor employing a tape of heat resistive material such as Teflon of polyimide resin. The tape is adhered to the thermistor assembly at a position between a heat roller and the thermistor to prevent the roller from being damaged.
As will be appreciated, it would be highly desirable to be able to easily replace a worn strap or tape that costs 37 cents rather than an entire thermistor assembly.
U.S. Pat. No. 5,350,896 granted to Amico et al on Sep. 27, 1994 discloses a heat and pressure fuser for fixing toner images to substrates. A dual heating lamp arrangement is utilized for elevating the surface temperature of a heated fuser roll. The operation of the dual heat lamps is controlled using dual thermistors for maintain steady state temperature fluctuations to .+-.3.degree. F. The thermistors are positioned, one adjacent the inboard end of the fuse and the other adjacent the outboard end thereof. The outboard thermistor is used to determine the desired on time of the fuser lamp and the inboard thermistor is used to determine which of the two lamps is on. If the inboard temperature is below a predetermined target value a 14 inch lamp is turned on. If the inboard temperature is at or above a predetermined target value an 11 inch lamp is turned.
U.S. Pat. application Ser. No. 08/295,375, U.S. Pat. No. 5,497,218, filed on Aug. 24, 1994 in the name of Amico et al discloses a thermistor calibration procedure wherein thermistors are calibrated by taking three thermistor resistance measurements at various temperatures. The resistance values are converted or transferred to Analog to Digital (A/D) bits or bit values which the machine's microprocessor can relate to temperature. The three values are utilized for constructing a calibration curve in the microprocessor for a specific thermistor bead. Use of the curve allows for mathematical interpolation by the microprocessor between calibration points, providing a wider and more accurate range to set the fuser roll temperatures. This increased accuracy over a wider range than before allows the sensor to become a common part for a family of products which may require different standby and runtime temperatures.
U.S. Pat. application Ser. No. 08/295,568, U.S. Pat. No. 5,497,218, filed on Aug. 25, 1994 in the name of Amico et al discloses a thermistor assembly wherein a wear resistant member is loosely disposed about a thermistor in contact with a thermistor bead such that it is between the bead and a heated fuser member and completely surrounds the components of the thermistor. The wear resistant member is in the form of a strap or tape having an opening adjacent one end through the opposite end is insertable. The inserted end is configured such that once it is inserted it can't be withdrawn through the opening. The strap is loosely supported in its operative position and can be easily removed, for example, by cutting through the strap and then withdrawing the strap from around the thermistor. Thus, a worn strap can be removed and a new one installed without discarding the thermistor and more importantly discarding the thermistor assembly.
U.S. Pat. No. 5,285,053 granted to Fowler et al on Feb. 8, 1994 discloses a control system and methods of making and operating the control system are provided, the system comprising an RTD temperature sensor, an applying unit operatively interconnected to the sensor for applying a varying voltage across the sensor and thereby providing a certain current level through the sensor for each certain voltage value being applied across the sensor while the sensor is sensing a certain temperature, and a microcomputer operatively interconnected to the sensor and the applying unit for determining from the voltage being applied across the sensor the actual temperature being sensed by the sensor, the applying unit comprising a charge pump arrangement.
U.S. Pat. Nos. 5,230,926, 5,164,161, 5,040,724, 4,971,452, 4,822,570 and 4,751,495, like the '053 patent, relate to RTD sensors.